1. Field of the Invention
The present invention relates to an electric power steering apparatus for assisting the force required to turn the steering wheel of a vehicle.
2. Description of the Related Art
An electric power steering apparatus has been developed which uses a vehicle speed sensor to detect the traveling speed of the vehicle and a torque sensor to detect the steering torque applied at the steering wheel. In the electric power steering apparatus of this type, when the steering torque measured exceeds a predetermined dead zone, a steering assistance motor is driven by a driving current of the magnitude determined according to the detected steering torque and the detected vehicle speed, and the rotational force of the motor is used to provide power to assist the force required to steer the vehicle, thereby providing a comfortable steering feeling to the driver. The steering assistance is controlled based on the operation by a control unit. One such electric power steering apparatus employs a dual controller system consisting of a first and second control units to ensure the system safety against malfunctions, such as runaway, of the controller.
With the electric power steering apparatus with the dual controller system, the first control unit serves to control a motor for providing an assistance force to steer the steering wheel by carrying out an operation to find a driving current value for the motor based on the detected torque and the detected vehicle speed. Such a control by the first control unit for the steering assistance tends to be executed after the first control unit has effected a diagnosis such as a motor-lock detection, in which the motor is put in motion independent from any input of the detected torque and vehicle speed, for an initial diagnosis of the operation of the system at the start of the first control unit.
On the other hand, the second control unit acts to detect abnormality of the first control unit with procedure steps of performing the same operation as the first control unit, and comparing the result of the operation with that of the operation by the first control unit in order to monitor inputs and outputs to and from the first control unit. Such a detection of abnormality of the first control unit by the second control unit is started concurrently with the start of operation of the first control unit.
This electric power steering apparatus is arranged such that if any abnormality is detected by the second control unit in the first control unit, the second control unit will make a fail-safe control, i.e. by stopping the steering assistance motor in order to assure the safety of the system.
In the conventional electric power steering apparatus, however, there is a risk of the second control unit taking erroneously as the abnormality of the first control unit the rotation of the motor trially effected independent from the input as the stage of the initial diagnosis as executed by the first control unit, on the ground that the detection by the second control unit is started at the same time when the first control unit starts its operation.